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The Earth's rotation speed can be influenced by many things and can both be sped up and slowed down. For example, the Sumatran earthquake in 2004 shortened the rotation period by 3 milliseconds. In theory negative leap seconds could be added but none have yet.

This gives rise to problems for computers since they are irregular and unpredictable, and can't be made part of algorithms. In extent, to properly calculate a precise time difference (actual elapsed time) over several years, you actually need to consult a table of added, historic leap seconds and take these into account too...